There are a number of known methods to recover carrier signal modulation. Some of the more well known methods include the Costas loop and the standard FM frequency discriminator. The Costas loop is primarily intended for the recovery of binary phase shift modulated signals on a synchronous basis. It is basically comprised of two parallel synchronous demodulators, whose local oscillator signals are in phase quadrature to each other. The outputs of the two signals are fed to a mixer, which provides an output that is a function of the phase difference between the mixer inputs. The mixer output signal is used as a control signal to adjust the local oscillator phase so as to keep the oscillator synchronous with the carrier of the received signal.
The FM frequency discriminator produces an output voltage which is directly proportional to the frequency deviation from its center frequency. A typically used configuration of an FM frequency discriminator comprises two selected resonant circuits whose output magnitudes are a function of the frequency of the signal applied. Two AM wave detectors are used to recover the magnitude of these signals. Each output has a magnitude with opposite polarity. The outputs of the AM wave detectors are added such that a linear DC signal is obtained which corresponds to a difference between the input signal frequency and the frequency to which the circuit is tuned.
Both the Costas loop and the standard FM frequency discriminator are practical for recovering their respective digital and audio modulated signals. However, the Costas loop has limited use for audio modulation, while the standard FM frequency discriminator does not have a stable or controllable center frequency. Since the standard frequency discriminator circuitry is generally not very stable and is highly sensitive to temperature variations, the tuning of the discriminator is not very reliable.
Furthermore, for receiver circuitry which is required to demodulate a signal having a very narrow bandwidth, the ability to maintain a stable center frequency is even more critical. Narrow band applications usually include an automatic frequency control (AFC) circuit, which includes compensating the center frequency of the local oscillator by a signal derived from the discriminator, as is similarly performed in the Costas loop. If the discriminator's center frequency is not stable the ability to accurately adjust the local oscillator is substantially degraded. Since narrow band applications are becoming more and more popular, the need for a low cost discriminator circuit having a stable center frequency is well recognized.
Although there have been other attempts to design accurate discriminator circuits, most are variations of the above mentioned techniques having similar type problems.